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The youngest children in a class are more likely to be diagnosed with ADHD than older children in the same class, a new study finds, and in some cases may not deserve the diagnosis.

Researchers led by Richard Morrow, a health research analyst at the Therapeutics Initiative at University of British Columbia, looked at ADHD diagnosis rates depending on whether children were born right before or after the school enrollment cutoff date. In British Columbia, the cutoff date for kindergarten or first grade is Dec. 31, which means that kids born in December are the youngest in their class, while those born in January are the oldest.

The researchers found that children born in December were 39% more likely to be diagnosed with ADHD and 48% more likely to be receiving medication to treat it than children in the same class born in January. In the study, which included data on 937, 943 children aged 6 to 12 over an 11-year period, Morrow and his colleagues also found that the rate of ADHD diagnoses increased steadily with each successive month from January to December.

The fact that there was such a difference in the rate of ADHD diagnoses simply based on children’s birthdates, all other things being equal, strongly suggests that less mature students may have been inappropriately being labeled with an attention deficit disorder. “What is clear from our study is that younger children in an classroom are more likely to receive a diagnosis of ADHD and drugs to treat that ADHD,” says Morrow. “But their relative maturity should come into play. Something to keep in mind when we look at behavioral problems is whether the behavior relates to differences in age and maturity.”

The effect was found in both boys and girls — even more pronounced in girls, although boys have higher rates of ADHD in general. In this study, boys born in December were 30% more likely to be diagnosed with ADHD than boys born in January,and 41% more likely to be prescribed a drug for the condition. Among the girls, those born in December were 70% more likely to be diagnosed and 77% more likely to be prescribed an ADHD treatment.

Part of the problem, says Morrow, is that there is no objective test or criteria for ADHD. The diagnosis is based on assessments by many people — parents, family members, teachers and others — about whether a child’s behavior conforms to accepted norms for his age. There’s also no consensus on how children of different ages should act, and they certainly develop at different rates.

Previous studies have found that younger children in a class have lower academic achievement and lower graduation rates than older children. But this is among the first studies to establish an association between children’s age and their likelihood of being diagnosed with a medical condition.

Some previous data support the new findings, however. A 2010 U.S. analysis that included various states with different school enrollment cutoffs found that rates of ADHD diagnoses were highest among children who were born in the month before their state’s cutoff date. In states enrolling students for the coming school year who were born before September, for example, ADHD rates were highest among those born in August; in states where the birthday threshold was December, diagnoses were highest among children born in November.

When it comes to ADHD, Morrow and his team suggest that doctors may not be emphasizing enough the importance of gathering complete information about a child’s behavior, particularly at school. If he is a year younger than his classmates, it’s unfair to compare his responses and peer interactions with those of older students — not to mention expecting him to act like the older kids — without accounting for the age difference.

Further, it’s not appropriate to medicate young children based on these comparisons, especially since ADHD drugs, while relatively safe, do have side effects, including changes in sleep, appetite and growth. Labeling youngsters with an unwarranted diagnosis of ADHD can also cause social and psychological problems, Morrow says, since parents and teachers may treat students with the disorder differently.

To address the misdiagnosing bias, Morrow says doctors should pay more attention to child evaluations not just in school, but in other settings as well, such as the home, on sports teams, at church or in other social environments. This is already part of the current guidelines for diagnosing ADHD, but many doctors probably place greater weight on school assessments because children spend most of their time in the classroom.

Physicians should also consider a child’s age when evaluating him for ADHD, Morrow says, and remember that not every deviation from so-called normal behavior is a sign of a medical condition. “It’s good to do everything we can to avoid medicalizing the normal range of childhood behavior,” says Morrow. “Differences in behaviors can arise because of differences in age or just from the fact that children mature at naturally different rates.” In other words, sometimes when kids get distracted or seem restless, they might just be being kids.